1. Field of the Invention
The present invention relates to a magnetic ferrite film for magnetic devices, in particular, to that with improved adhesiveness to Si substrates.
2. Description of the Related Art
Small-sized and lightweight portable appliances capable of being driven by the power of batteries are in great demand. For application to multi-media information services, they are much desired to have advanced functions including communication and display functions, and also to have the capabilities of rapidly processing large-scale information media including images, etc. With these, there is much increasing a great demand for power sources capable of converting a single voltage from batteries to a plurality of different voltages applicable to different types of devices such as CPUs, LCD modules, power amplifiers for communication, etc. For small-sized and lightweight portable appliances with such advanced functions, the important theme is to realize small-sized, high-efficiency power sources.
In that situation, much used are DC-DC converters in which the direct current inputted is intermittently controlled by the action of a semiconductor switch to give a stable, desired voltage to be outputted from it. In the Journal of the Applied Magnetics Society of Japan, Vol. 20, No. 5, 1996, p. 922, described is a power source with a plane inductor mounted thereon, in which the inductor comprises a thin magnetic film. In Japanese Patent Laid-Open No. 134820/1997, disclosed is a plane magnetic device (e.g., inductor, etc.) with a plane coil being sandwiched between a soft magnetic substance via an insulator therebetween, in which the plain coil conductor is composed of a plurality of conductor lines divided in different sections. These are suitable to thin devices, and are expected to be used in the field of portable appliances and others that are required to be small-sized and lightweight.
However, the conventional plane inductors are produced by forming a magnetic metal film of from 6 to 7 xcexcm thick on Si substrates through sputtering or the like. Therefore, as compared with those for other conventional inductors with a conductor line wound around a sintered ferrite core, the production costs for such plane inductors inevitably increase, and it is a bar to industrialization of the plane inductors. To solve this problem, we, the present inventors have already proposed a technique of substituting the magnetic metal film in those plane inductors with a magnetic ferrite film to be formed through printing or the like (see Japanese Patent laid-Open No. 26239/1999).
However, the technique requires further improving the adhesiveness of the magnetic ferrite film to Si substrates. If its adhesiveness thereto is not good, the magnetic ferrite film will peel away from Si substrates in the process of fabricating magnetic devices comprising the film-coated substrate, and the reliability of the devices fabricated will be thereby lowered.
In that situation, the present invention is to increase the adhesiveness of a magnetic ferrite film to Si substrates thereby to improve the reliability of thin-film magnetic devices comprising the film-coated substrate.
Specifically, the invention is a magnetic ferrite film formed on an Si substrate for magnetic devices, in which the areal ratio of the Sixe2x80x94Cu rich phase to the magnetic film, at least to the part thereof directly adjacent to the surface of the Si substrate is at most 50%, or the CuO content of the magnetic film at least in the part of the film directly adjacent to the surface of the Si substrate is at most 5 mol %.
To solve the problems noted above, the CuO content of the part of the magnetic film directly adjacent to the Si substrate shall be at most 5 mol %. The reason is as follows: We, the present inventors have assiduously studied the adhesiveness of magnetic ferrite films to Si substrates and have found that a Cuxe2x80x94Si rich deposit phase, when formed in the interface between the magnetic film and the Si substrate through the reaction of Cu in the magnetic film with Si of the substrate, significantly lowers the adhesiveness between the magnetic film and the Si substrate. On the basis of this finding, we have reached the conclusion that reducing the Cuxe2x80x94Si rich deposit phase in the interface between the magnetic film and the Si substrate results in the increase in the adhesiveness therebetween. For this, the CuO content of the magnetic ferrite film at least in the part of the film existing around the interface between the film and the Si substrate must be at most 5 mol %. If it is larger than 5 mol %, the Cuxe2x80x94Si rich deposit phase will be much formed around the interface between the magnetic film and the Si substrate, thereby lowering the adhesiveness therebetween. Therefore, in the invention, the CuO content of the magnetic ferrite film around the interface between the film and the Si substrate is defined to be at most 5 mol % throughout the film; or it may be defined to be at most 5 mol % only in the area around the interface between the film and the Si substrate, while in the other area the CuO content could be over 5 mol %.
To produce the magnetic ferrite film of that type, for example, a first ferrite layer having a CuO content of not larger than 5 mol % is printed on an Si substrate, and the second and other ferrite layers having a CuO content of larger than 5 mol % are printed over the first layer. In this process, the thickness of the first layer (having a CuO content of not larger than 5 mol %) varies, depending on the printing condition employed, but may be generally at most about 1 xcexcm or so. In practice, therefore, a ferrite layer having CuO content of at most 5 mol % shall be realized within the range of up to about 1 xcexcm from the interface between the layer and the Si substrate.
So far as the CuO content of the magnetic ferrite film is not larger than 5 mol % in the interface between the film and the Si substrate, good adhesiveness of the film to the substrate could be ensured. In this case, preferably, the magnetic film is of a spinel ferrite comprising from 40 to 50 mol % of Fe2O3, from 15 to 35 mol % of ZnO, from 0 to 20 mol % of CuO, and from 0 to 10 mol % of Bi2O3 with NiO and inevitable impurities as the balance, for the average composition of the entire film. The reason why the composition of the magnetic film is defined as above is mentioned below. Fe2O3, from 40 to 50 mol %:
If Fe2O3 is over 50 mol %, Fe2+ ions existing in the film will greatly lower the electric resistance of the film. The reduction in the electric resistance of the film shall increase the ferrite core loss. On the other hand, if Fe2O3 is below 40 mol %, the inductance of the film will be greatly lowered. Therefore, Fe2O3 is defined to fall between 40 and 50 mol %. ZnO, from 15 to 35 mol %:
ZnO has great influences on the inductance and the Curie temperature of the film. Preferably, the Curie temperature of the film is not lower than about 120xc2x0 C. If ZnO is below 15 mol %, the inductance of the film will be lowered even though the Curie temperature thereof could be high. On the other hand, if ZnO is above 35 mol %, the Curie temperature of the film will be low even though the inductance thereof could be high. Therefore, ZnO is defined to fall between 15 and 35 mol %. CuO, from 0 to 20 mol %:
CuO is added to the film so as to lower the temperature at which the film is sintered. The CuO content of the film is defined to be at most 5 mol % in the area around the interface between the film and the underlying Si substrate, as so mentioned hereinabove. In the other area, it is desirable that the CuO content of the film falls between 0 and 20 mol %. If CuO is above 20 mol %, it could lower the sintering temperature but will reduce the inductance of the film. Therefore, the uppermost limit of CuO is defined to be 20 mol %. If the sintering temperature does not need to be lowered, no CuO may be added to the film. Therefore, except for the inevitable amount thereof, the lowermost limit of CuO could be 0%. In this case, adding CuO to the area around the interface between the film and the substrate is unnecessary. Bi2O3, from 0 to 10 mol %:
Like CuO, Bi2O3 also acts to lower the sintering temperature. If Bi2O3 is above 10 mol %, it could lower the sintering temperature but will reduce the inductance of the film. Therefore, the uppermost limit of Bi2O3 is defined to be 10 mol %. If the sintering temperature does not need to be lowered, no Bi2O3 may be added to the film.
Actual synthetic products of ferrite generally have a complicated structure comprising plural metal ions of different valences. In the present invention, however, the metal ions constituting ferrite are expressed as their oxides, concretely as Fe2O3, NiO, ZnO and CuO in which Fe is a trivalent ion, and Ni, Zn and Cu are divalent ions.
The method of producing the magnetic ferrite film of the invention is not specifically defined. Preferably, the film is produced by mixing a ferrite powder having been previously prepared to have a predetermined composition, with a binder such as ethyl cellulose or the like to give a paste, then applying the resulting paste onto an Si substrate, and sintering it at a temperature falling between 920 and 1250xc2x0 C. While the ferrite powder is mixed with a binder, a solvent such as butylcarbinol, terpineol or the like may be added thereto, if desired. The method of applying the paste onto an Si substrate is not also specifically defined, including, for example, screen printing, doctor blade coating, etc. On the surface of the ferrite film thus formed in the manner as above, a plane-structured coil pattern is formed through metal plating or the like; and another magnetic ferrite or metal film is formed over the patterned surface. In that manner, magnetic devices such as transformers, inductors and others comprising the magnetic ferrite film of the invention are fabricated.